Distance-measuring equipment (DME) is currently installed on aircraft for use with a single antenna mounted under the fuselage. In some situations, the lower mounted DME antenna may not provide adequate communication with a DME ground station because line of sight to the DME ground station is compromised. The lack of line of sight can be due to the attitude and orientation of the ownship (e.g., the ownship has turned so that the lower antenna is turned away from the DME ground station or the landing gear doors provide an obstacle).
Also, use of the lower mounted DME antenna might not be possible, due to a failure in the antenna or cabling from the avionics to the antenna.